Poly(hydroxyalkanoic acid) (PHA) polymers such as poly(lactic acid) (PLA) can be polymerized from renewable sources rather than petroleum and are compostable. They have a broad range of industrial and biomedical applications. However, physical limitations such as brittleness and slow crystallization may prevent easy thermoforming of PHAs into articles that have an acceptable degree of toughness and thermal stability for many applications. Extruded amorphous sheeting may also be too brittle for handling in continuous moving equipment without breakage.
To make thermoformed articles, a PHA resin, such as PLA, is first extruded into an amorphous sheet and then formed at an optimal temperature and speed into a semicrystalline container such as a cup. The thermoforming speed and sheet-forming temperatures are optimized for a specific cup design. High sheet-forming pre-heat temperatures can cause the pre-formed sheet to deflect and fall before being formed. Too low a forming temperature can give a sheet of high stiffness that is unable to be physically formed into a deep cup. High thermoforming speeds can rupture the sheet before it achieves the shape of the cup. Too low a forming speed can allow the cup to begin crystallizing during forming, which can lead to unacceptable haze or can result in the cup not achieving the full depth of formation.
The narrowest operating window of temperature and forming speed is for cups having the greatest degree of formation, such as cups with high height-to-diameter ratios. At forming speeds and temperatures useful for large-scale manufacturing of thermoformed articles, thermoformed articles of non-modified PLA may not have high use-temperatures because the formed article may have regions that are incompletely crystallized. Such cups may be more highly crystalline in those regions that were oriented during forming, such as the walls, and of lower or no crystallinity in those regions having low degrees of orientation, such as the base or rim. Because nonmodified PLA is often slow to crystallize, the resin in the resulting cup that is thermoformed at practical high speeds may not be everywhere fully crystallized. The regions not fully crystallized may either soften at the glass transition temperature (Tg) or may experience slow crystallization and subsequent shrinkage when exposed to higher temperatures. Since nonmodified PLA typically has a Tg of around 55° C., the use-temperature of the cup is limited to about 55° C. This is undesirably low because the containers may experience temperatures of 65° C. or more during normal shipping and handling. In addition, hot-filling of containers is typically carried out at about 80° C. or above. The PLA cups that are not fully crystallized may also deform and stick together at temperatures above the Tg.
The use-temperature of thermoformed articles can be raised by annealing the articles in their molds. Annealing is carried out most effectively at temperatures between the Tg and the melting range of the resin composition, allowing the composition to crystallize. Annealing would greatly increase the cost for making cups of nonmodified PLA by increasing cycle time, decreasing throughput and through higher energy costs associated with the annealing.
It is desirable to obtain toughened PHAs that are easily thermoformed into a variety of articles with an acceptable level of thermal stability, preferably without the need for annealing. Modification of the PHAs by addition of other resin materials may improve toughness and crystallization rates during thermoforming.
Patent Applications US200410242803 and WO03/014224 disclose miscible blends of poly(lactic acid) with polyacetal resin that may contain impact modifiers. U.S. Pat. No. 6,943,214 discloses blends of polylactic acid and polyoxymethylene toughened with random ethylene copolymers comprising glycidyl groups.